[JS] JsExport diagnostics and legacy support

Account for JsExport in legacy backend namer. It means we
catch overloaded exported function conflicts for free!

Add error diagnostics:
* NESTED_JS_EXPORT (Fixes KT-36798)
* WRONG_EXPORTED_DECLARATION (Part of the fix for KT-37752)
* NON_EXPORTABLE_TYPE (Fixes KT-37771)

js/js.translator/testData/box/char/topLevelCallables.kt

@@ -1,16 +1,15 @@
 // SKIP_MINIFICATION
+
+// Exported declaration uses non-exportable return type: Char
 // IGNORE_BACKEND: JS_IR
+
 @JsName("foo")
-@JsExport
 fun foo(): Char = '1'
 
-@JsExport
 val p1: Char = '2'
 
-@JsExport
 var p2: Char = '3'
 
-@JsExport
 var p3: Char = '4'
     get() = field + 1
     set(value) {